Optical refrigerators are solid-state devices that produce cooling by the interaction of substantially monochromatic light with selected materials. Earlier examples of this technology employed ytterbium-doped and thulium-doped glass and crystals to provide this optical cooling. Others have used laser-dye solutions. However the efficiency and operating temperature of these devices limited their application. Recently, researchers have explored the possibility of basing optical refrigerators on semiconductor materials.
Semiconductor-based optical refrigerators rely on the excitation of sub-thermal electron hole pairs. To accomplish this, a laser is tuned to a wavelength just below the edge of the bandgap, known as the Urbach tail, of the semiconductor, exciting these electron hole pairs. In only approximately 10xe2x88x9212 seconds, the excited free carriers thermally equilibrated by absorbing phonons, thereby cooling the semiconductor. Then, in approximately 10xe2x88x929 seconds, the excited free carriers recombine, and emit photons of higher energy than the photons received from the laser. Thus, the escaping fluorescent photons remove both the pump photon energy and the energy of the absorbed phonons. This re-emission of photons with higher energies than of the absorbed photons is called anti-Stokes fluorescence. The beneficial net result of this process is the removal of heat from the material.
In fact, semiconductor materials offer numerous advantages over the earlier rare-earth-based optical refrigerators. First, semiconductor materials interact with laser radiation much better than does ytterbium-doped glasses or crystals. Due to this fact, the semiconductor materials can be much more compact, less than a millionth of the active mass of ytterbium-based devices. Second, semiconductor fabrication technology is quite mature, allowing high-purity devices to be fabricated that exhibit very little parasitic heating. This low parasitic heating may allow a semiconductor-based optical refrigerator to operate at lower temperatures than ytterbium-based devices. Semiconductor-based devices could operate at temperatures as low as 10xc2x0 Kelvin. Third, semiconductor-based devices can be mass-produced at much lower cost than the ytterbium-based devices. Another potential benefit of semiconductor-based optical refrigerators is that the pumping laser diode could be located on the same substrate as the refrigerator, further increasing the compactness and lowering the cost of semiconductor-based optical refrigerators.
However, with all the potential benefits of semiconductor-based optical refrigerators, there has been one significant technological difficulty that has prevented them from being realized. This difficulty relates to the fact that semiconductor materials generally have large indices of refraction that prevents the re-emitted fluorescent light from escaping quickly. Experimentation has shown that the interaction of the laser light with the semiconductor material does locally cool the material. However, most of the fluorescent light leaving the interaction region is internally reflected at the outer boundary of the semiconductor material because of its large index of refraction and is subsequently reabsorbed in the semiconductor. This action by the semiconductor material is termed xe2x80x9cradiation trapping.xe2x80x9d
The repeated absorption, fluorescence and reabsorption heat the semiconductor material and overwhelm the optical refrigeration effect. Semiconductor materials have not yet shown net cooling, where the overall temperature of the material exhibits a net temperature drop.
The present invention overcomes this problem with semiconductor-based optical refrigerators, allowing efficient optical refrigerators to be made using semiconductor materials. The invention accomplishes this with a novel arrangement of materials that facilitates emission of the fluorescence from the semiconductor material in an efficient manner.
It is therefore an object of this invention to provide an optical refrigerator based on a semiconductor material.
It is another object of the present invention to provide a semiconductor-based optical refrigerator that efficiently utilizes the input laser energy.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a semiconductor based optical refrigerator cooled by optical radiation from a diode laser comprises a cooling layer having a first and second end, with the first end in optical contact with the optical radiation for producing fluorescent photons in response to excitation by the optical radiation, the cooling layer defining first and second faces, with a first passivating layer having first and second faces, the first face in contact with the first face of the cooling layer for carrying fluorescent photons away from the cooling layer, and the second face in contact with a device to be cooled. A second passivating layer having first and second faces, has the first face in contact with the second face of the cooling layer for carrying fluorescent photons away from the cooling layer. An absorbing layer is spaced apart from the second face of the passivating layer for receiving the fluorescent photons from the second optical conduction layer and producing thermal phonons. A heat sink is in contact with the absorbing layer for dissipating heat from the thermal phonons.
In another aspect of the present invention and in accordance with its purposes and objectives, a semiconductor based optical refrigerator cooled by optical radiation from a diode laser comprises a cooling layer having a first and second end, with the first end in optical contact with the optical radiation for producing fluorescent photons in response to excitation by the optical radiation, the cooling layer defining first and second faces, and a first passivating layer having first and second faces, the first face in contact with the first face of the cooling layer for carrying fluorescent photons away from the cooling layer, and the second face in contact with a device to be cooled. A second passivating layer having first and second faces, with the first face in contact with the second face of the cooling layer for carrying fluorescent photons away from the cooling layer. An absorbing layer is spaced apart from the second face of the passivating layer for receiving the fluorescent photons from said the optical conduction layer and producing thermal phonons. An enclosure encloses and is in close proximity to the device to be cooled, the cooling layer, the first and second passivating layer, and the absorbing layer. The enclosure is made of a layer of anti-reflective material on a layer of absorbing material on a layer of gold on a heat sink and defining an aperture though said heat sink and through the layer of gold, the layer of absorbing material, and the layer of anti-reflective material. Wherein a laser beam emitted through the aperture cools the cooling layer providing cooling of the device to be cooled.
In yet another aspect of the present invention and in accordance with its purposes and objectives, an optical refrigerator using ytterbium-doped glass having first and second faces as a cooling layer comprises first and second dielectric mirrors deposited onto the first and second faces of the ytterbium-doped glass with a device to be cooled deposited onto the first dielectric mirror. An enclosure encloses and is in close proximity to the ytterbium-doped glass, the first and second dielectric mirrors, and the device to be cooled, the enclosure being made of a layer of anti-reflective material on a layer of absorbing material on a layer of gold on a heat sink, and defining an aperture though the heat sink, the layer of gold, the layer of absorbing material, and the layer of anti-reflective material. Wherein a laser beam emitted through the aperture cools the cooling layer providing cooling of the device to be cooled.